It is known in the prior art to recover elemental sulfur from hydrogen sulfide containing gas streams as is set forth in the article "Fundamentals of Sulfur Recovery by the Claus Process" by B. Gene Goar, published in the 1977 Gas Conditioning Conference Report.
It is also known to use oxygen enrichment in the operation of a Claus sulfur plant in order to increase the capacity of hydrogen sulfide handled as well as the total throughput of the plant as set forth in the article "Oxygen Use in Claus Sulfur Plants" by M. R. Gray and W. Y. Svrcek published in the 1981 Gas Conditioning Conference Report. In that article it was disclosed that oxygen can be added to the air feed to the burner of the reaction furnace in order to increase the amount of hydrogen sulfide which is combusted to sulfur dioxide for later catalytic conversion with additional hydrogen sulfide to the elemental liquid sulfur product of the Claus process. The article recites that the maximum capacity increase which can be achieved with oxygen enrichment is determined by the pressure drop through the plant and the reactor space velocities. However, a further limitation set forth in the article is that for a given plant stream, temperatures and sulfur condenser capacity may limit the potential capacity increase using oxygen enrichment. Specifically, stream temperatures in the reaction furnace and in the converter beds may increase due to oxygen enrichment and in fact such increase from oxygen enrichment reaches the maximum tolerable temperature of the materials used in such a furnace, namely the refractory lining.
In the 1983 publication by Linde of Union Carbide entitled "Claus Plant Oxygen Enrichment", it is noted that oxygen-enrichment limitations exist for rich H.sub.2 S streams due to temperature limits in the furnace or waste heat boiler of a Claus plant.
U.S. Pat. No. 3,822,341 describes a Claus plant using oxygen enrichment in which one source of oxygen is initially used to strip residual SO.sub.2 from a sidestream in vessel 92 before the oxygen stream in line 96 is optionally recycled to be included with the oxygen in line 12 going to the combustion zone of the waste heat boiler 8. As recited at col. 5, lines 65-68 of the specification. Because the oxygen content of such a stream is completely consumed in an exothermic reaction this stream can not be utilized as a moderating medium for the flame temperature of the reaction furnace. As described by Goar, Claus sulfur plants typically have an adiabatic reaction furnace followed by a waste heat boiler. The excessive temperature problem with oxygen enriched operation occurs in the adiabatic furnace. U.S. Pat. No. 3,822,341 ignores the existence of this problem.
U.S. Pat. No. 4,153,674 discloses a Claus plant and tailgas cleanup plant wherein a gas stream in line 20 is removed from the tailgas system and is returned or recycled to the front end of the Claus plant 7. This patent does not consider oxygen enrichment or flame temperature moderation by a recycle stream. Also the tail gas is reacted to convert all sulfur to hydrogen sulfide which is absorbed, stripped and returned to the Claus plant.
The present invention overcomes the shortcomings of the prior art by increasing throughput of a Claus plant by oxygen enrichment to an extent beyond that contemplated in the prior art due to flame temperature limitations. This is achieved by the recycle of an intermediate gas stream from the first condenser in the Claus train to moderate the temperature in the flame of the burner of the reaction furnace.